Naturally-occurring poly(.beta.-hydroxyalkanoates) (abbreviated as PHA) are a class of biodegradable polymers which have been the subject of increasing interest as a material which can be employed in disposable articles and specialty medical products. Generally, these polymers exhibit biologically-mediated environmental degradability, hydrolytic degradation and have been shown to possess advantageous characteristics when used in vivo. Among the most well-known PHA's are homopolymers of poly(.beta.-hydroxybutyrate) (abbreviated as PHB). See, for example, Brandl et al., Adv. in Biochem. Eng./Biotech., 41:77 (1990); Doi, Y., Microb. Polyesters, VCH Publishers: New York (1990); and Steinbuchel et al., Mole. Microb., 5(3):535 (1991).
However, the rate of biodegredation of PHA can be affected by altering the stereochemistry of the polymer. For example, crystalline, naturally-occurring PHB and its synthetic analogue, synthesized by ring-opening [R]-.beta.-methyl .beta.-propiolactone, have been prepared. These PHB's are composed of only [R]-enantiomers, wherein the .beta.-methyl substituent groups are arranged in a stereoregular isotactic repeat unit sequence. This polymer structure generally exhibits relatively rapid rates of biodegredation in a variety of disposal environments. However, synthetic PHB's, which are composed of 50% [R]- and [S]-repeat units, where the .beta.-methyl substituent groups are arranged in an atactic configuration, have shown considerably slower degradation kinetics. In fact, the sustained biodegradability of atactic PHB, beyond initial surface degradation, is questionable at present. Furthermore, 50% [R] isotactic PHB, which has blocks of [R]-PHB and [S]-PHB chain segments, has thus far shown poor degradation characteristics.
In addition, methods for forming synthetic 50% [R]-PHB have produced atactic polymers of high molecular weight, as well as low molecular weight polymers with partial stereoregulation, whose range of useful applications are significantly limited. See, for example, Kricheldorf et al., Macromolecules, 24:1944 (1991).
Therefore, a need exists for substituted-poly(.beta.-propioesters), and a method of forming such polymers, which overcome the above-mentioned problems.